Sheet materials, and in particular metal sheet materials, are often processed by machines that apply pressure to the sheets to reduce their thickness, or apply an abrasive surface to the sheets to provide a desired surface finish thereon. For example, the rolling mill illustrated in FIG. 1 may be used to transform a thick sheet into a thin sheet through the application of pressure by opposed rollers. The surface finishing machine illustrated in FIG. 2 may be used to apply a rotating abrasive surface to a sheet to provide a refined surface finish, for example. Although reduction rolling (as described with reference to FIG. 1) and abrasive finishing (as described with reference to FIG. 2) are distinct processes, the workpieces that emerge from either process often exhibit a similar defect known as "optical chatter".
Optical chatter is defined as a pattern of visible, alternating dark and light bands on the surface of the workpiece. The spacing of the bands is usually linked to the speed of one of the rotating elements of the apparatus, and to the feed rate of the workpiece through the apparatus. Optical chatter is distinguishable from "physical chatter" in that physical chatter represents a measurable variation in the size of the workpiece, whereas optical chatter is a visible defect with surface characteristics that are difficult or impossible to measure accurately. Physical chatter has typically been remedied by refining machine components to obtain perfectly round rollers, bearings with extremely small tolerances, and the like. Optical chatter, however, has not been adequately remedied. In fact, optical chatter may even occur in workpieces finished on machines exhibiting no detectable departures from optimum machine performance, and neither physical, optical, nor chemical analysis of the workpiece has adequately explained the cause of optical chatter. Thus, even well designed and engineered finishing machines can produce optical chatter marks on a workpiece.
The generally accepted explanation for optical chatter is that it is caused by oscillatory movement of the roller away from and toward (i.e. in a plane generally perpendicular to) the workpiece that is being finished. See, for example, Identification of Chatter Sources in Cold Rolling Mills, Gerald L. Nessler et. al., Iron and Steel Engineer, January, 1993, and Analysis of Chatter Vibration Phenomena of Rolling Mills Using Finite Element Analysis, Remn-Min Guo et. al. , Iron and Steel Engineer, January, 1993. Thus, when the workpiece is fed into a machine in a horizontal plane, as shown in FIGS. 1 and 2, the oscillatory roller movement that causes the optical chatter is thought to be in the vertical plane. This is described herein as "vertical roller oscillation." Vertical roller oscillation is thought to have been caused by radial forces produced by either roller imbalance, or out-of-roundness of the abrasive roller or the back-up roller.
To provide a better understanding of typical processes that may produce workpieces exhibiting optical chatter marks, reference is made to FIGS. 1 and 2. In FIG. 1, rolling mill 20 is used to reduce the thickness of a workpiece (such as a metal sheet) to a predetermined thickness, and includes a powered roller 24 and a backup roller 26. The powered roller 24 includes a roller cage 28 that supports a plurality of working rollers 30, which are free to rotate within the roller cage 28. The powered roller 24 is rotated about central axis 34 in the direction indicated by arrow 32.
The backup roller 26 is adjacent to and spaced from the powered roller 24, and the spacing is less than the initial thickness of the workpiece. The backup roller 26 includes a roller cage 28 that supports a plurality of working rollers 30. The working rollers 30 rotate in the directions indicated by arrows 40 and 42. The backup roller 26 rotates about central axis 38, as indicated by arrow 36, such that the respective peripheral surfaces of the powered roller and the backup roller propel workpiece 22 in direction D.
In operation, workpiece 22 is fed into mill 20 between the powered roller 24 and the backup roller 26 from the left and emerges with reduced thickness to the right. Because the space between the rollers is less than the thickness of the workpiece, the rollers compress the workpiece and the workpiece undergoes plastic deformation with a resultant reduction in thickness. Workpiece 22 remains in rolling contact with powered roller 24 and backup roller 26 as it passes through mill 20, with an insignificant amount of relative motion, or slippage, in the areas of mutual contact. An effect of the plastic deformation of the workpiece 22 is to emboss any surface defects present in either the powered roller 24 or the backup roller 26 onto the surface of the workpiece 22. Thus, the surfaces of the rollers are typically very smooth, and are often finished with a grinding apparatus that imparts a very fine finish to the surfaces of rollers. However, even when smooth rollers are used, optical chatter marks may still be present, which was thought to be a product of vertical roller oscillation by rollers 24 or 26 or both, as described above.
FIG. 2 illustrates a surface finishing machine 40 that imparts a surface finish to a workpiece 42. Workpiece 42 may be a single sheet, as depicted, or a continuous coil of material, for example. A powered roller 44 is rotationally driven about central axis 50 to rotate an abrasive belt 46 in the direction indicated by arrow 48. Generally, the rotation direction is described as being either "against the feed" or "with the feed." Arrow 48 depicts "with the feed" rotation, although the rotational direction of the rollers could be reversed to provide "against the feed" rotation, if desired. An idler roller 52 is spaced from the powered roller 44 to maintains tension in abrasive belt 46. In an alternative embodiment, an abrasive may be provided on the exterior surface of the powered roller 44, which would eliminate the need for the abrasive belt 46 and the idler roller 52.
Workpiece 42 is fed at a controlled rate into surface finishing machine 40. The feed rate is controlled by a feed mechanism 54, which includes a powered feed roller 56, an idler feed roller 58, and a belt 60. Powered feed roller 56 is rotationally driven to drive belt 60 as indicated by arrow 64, and maintains tension in belt 60. Belt 60 has an exterior surface that frictionally engages workpiece 42, to prevent workpiece 42 from slipping with respect to belt 60. A backup roller 66 is disposed beneath belt 60 and proximate the lower radius of powered roller 44, to provide a compressive surface that holds the workpiece 42 in contact with powered roller 44.
The feed rate of workpiece 42 into surface finishing machine 40 is less than the surface speed of powered roller 44, such that there is relative motion (and a corresponding abrasive action) between the workpiece and abrasive belt 46. The abrasive media thus provides a surface finish on the workpiece 42, consisting of a large number of parallel scratches. Optical chatter marks are also produced by apparatuses such as the illustrated surface finishing machine, and it is believed that such optical chatter marks are produced by vertical roller oscillation of roller 44, roller 66, or both, as described above.
Because optical chatter is a visual surface condition that does not affect the structural integrity of most workpieces (sheet metal, for example), it has not in the past been perceived as a defect that required a great deal of attention. However, the demand for finished metal sheets for larger, visually important applications is growing. For example, large metal sheets might be used to adorn the exterior of a building. In such an application, optical chatter would be evident, and would greatly detract from the overall appearance of the building.
Accordingly, it would be a decided advantage to be able to apply a rollered finish to a metal workpiece economically, without also providing optical chatter marks that detract from the appearance of the workpiece.